This invention starts from a circuit arrangement for a reversible electric motor supplied from a voltage source, which motor serves for driving a door locking element in motor vehicles and, for locking and unlocking the vehicle door, may be switched on via an operating switch.
So-called central door locking systems including electric drive elements are normally mechanically built up in such a way that a motor vehicle door can be locked or unlocked both electrically and manually completely independently. However, this can involve problems when the locking condition of the door at the beginning of a central control of the electric motors does not correspond to the condition it had after the latest displacement of the door locking element by the electric motors, when an uneven number of manual operations had thus been effected.
So for instance from the German laid open print 2,854,713 a combined window lifter and central door locking system is known in which, for locking and unlocking of a vehicle door, the entire electric motor is swivelled round its driven shaft. If in such a system it is provided to adjust the door locking element manually it is, for instance, possible, to unlock the door manually after the door was locked by the electric motor previously. The door locking element is then in a position which corresponds to the unlocked door, but the electric motor occupies a position which corresponds to the locked door.
A circuit arrangement for a combined window lifter and central door lockiing system known from the DE-OS 2,854,713 is shown by the DE-OS 2,854,670. This circuit arrangement is laid out in such a way that the electric motor for opening and closing of the window in a door may be controlled through a reversing switch in the interior of the motor vehicle. When the window will be completely closed also the respective door will be locked, because a position switch assigned to the motor and operated by it will only change over upon termination of the locking operation. When the window will be opened at first the door will be unlocked, before the window pane will be moved. In addition thereto all electric motors can be controlled simultaneously through one or several operating switches in order to lock or unlock all doors, whereby prior to a locking operation all possibly open windows will be closed.
Now the following can happen. The driver unlocks all doors before he enters the vehicle. En route the driver opens the window of the driver's door by the electric motor and then closes it again. During this closing operation also the door is locked. Thus the electric motor in the driver's door occupies a position which corresponds to the locked door, whereas the electric motors in the other doors occupy positions which correspond to the unlocked doors. Before the driver gets off the vehicle he unlocks his door manually. The electric motor therefore retains the locking position. When the driver now wishes to lock the doors by the electric motors the driver's door remains unlocked, because the electric motor concerned cannot be swivelled further.
It is an object of the present invention to develop a circuit arrangement according to the present invention in such a way that it can be operated simply and ensures a high operational reliability. Thereby first of all it is to be ensured that after a locking command has been given all doors will be actually locked.